1. Field of the Invention
The present invention relates to a method of making a diffusion plate having desirable diffusion characteristics by forming an optical speckle pattern in a space and optically recording the speckle pattern. The optical recording process used in the method may involve recording with rays other than visible light rays.
2. Description of the Prior Art
To optically make a diffusion plate there are known various methods. According to one of those known methods a coherent light is projected on a diffusion plate to form a speckle pattern in the exit light beam from the plate and the speckle pattern is photographically recorded on a dry plate. The photographic dry plate is then subjected to a bleaching treatment to form a relief pattern while controlling the height of relief under a predetermined value. The diffusion plate thus produced is used for uniformly illuminating an object in holography (Japanese Patent Application Publication No. 17093/1973).
According to another known method, the light for photographing a diffusion plate is spatial frequency filtered in the light path to form another diffusion plate having rotationally asymmetric diffusion characteristics which is suitably used as a screen for a projector (Japanese Patent Application Publication No. 10265/1974). According to a further known method, the diffused light from a diffusion plate as an object light and a coherent reference light are superimposed on each other and recorded to form a holographic diffusion plate which is useful as a microfilm viewer (U.S. Pat. No. 3,708,217).
Also, the applicant of the present application has already disclosed that when a mask having a suitable opening therein is placed on the diffusion plate to be illuminated by a coherent light, the shape of the speckle pattern in the space wherein it is recorded can be well controlled and made uniform so as to give another diffusion plate having desirable diffusion characteristics (Japanese Patent Application Laid Open No. 51755/1978) and that the diffusion plate thus formed can be used as a very effective mat surface of a focusing plate in a camera finder optical system (Japanese Patent Application Laid Open No. 42726/1978).
All of the known methods described above are common in that as the means for generating a speckle pattern there is used an ordinary diffusion plate, that is, a diffusion plate having rotationally symmetric diffusion characteristics.